Ans is option (D)
Let there be $x$ ltr. of solution initially.
$\therefore$ Initial volume of A and B respectively: $\frac{7}{12}\times x, \frac{5}{12}x$ ltr.
Now, $9$ ltr. of solution is removed and the same volume of B is poured in the can.
$\therefore$ Volume of B removed from the can $=(\frac{\frac{5}{12}\times x \times 9}{x})=3.75$ ltr.
After pouring $9$ ltr. of B again, the volume of B in the can now $=\frac{9}{16}\times x$ ltr
$\therefore$ $(\frac{5}{12}\times x)-3.75+9=(\frac{9}{16}\times x)$ $\Rightarrow$ $x=36$ ltr
$\therefore$ Volume of A in the can initially $=\frac{7}{12}\times 36=21$ ltr